(a) Field of the Invention
The present invention relates to a process for preparing film by wet method, using a coagulation bath consisting of two kinds of liquids which form two phases.
(B) Description of the Prior Art
Reverse osmosis method is widely applied for purification, concentration, separation, etc., of a substance. In performing this method, it is essential to use an asymmetric film the structure of which is different between its surface and backside.
Heretofore, said film was prepared by a casting process which comprises coating a starting solution on a glass plate and forming a film by evaporating solvent gradually. For instance, Loeb film for converting sea water into fresh water can be obtained by the process of coating a dope solution consisting of cellulose acetate in such a good solvent as acetone or dioxane and a swelling agent (water or formamide) on the plate to make a layer of appropriate thickness, evaporating the major part or all of the solvent at a lower temperature, eluting the swelling agent and the residual solvent if any by immersing said layer and heating the layer at a temperature of 70.degree. to 90.degree. C.
However, on performing this casting process, heat treatment must be done under a temperature of lower than 80.degree. C. since an excellent film cannot be obtained by treatment at a higher temperature. Thus, a solvent applicable for the process is restricted and also the starting solution is restricted.